revert #3723, add special case for src attributes

src/compiler/compile/render_dom/wrappers/Element/Attribute.ts

@@ -73,10 +73,11 @@ export default class AttributeWrapper {
 		const dependencies = this.node.get_dependencies();
 		const value = this.get_value(block);
 
+		const is_src = this.node.name === 'src'; // TODO retire this exception in favour of https://github.com/sveltejs/svelte/issues/3750
 		const is_select_value_attribute =
 			name === 'value' && element.node.name === 'select';
 
-		const should_cache = is_select_value_attribute; // TODO is this necessary?
+		const should_cache = is_src || this.node.should_cache() || is_select_value_attribute; // TODO is this necessary?
 
 		const last = should_cache && block.get_unique_name(
 			`${element.var.name}_${name.replace(/[^a-zA-Z_$]/g, '_')}_value`
@@ -119,6 +120,11 @@ export default class AttributeWrapper {
 				${last} = ${value};
 				${updater}
 			`);
+		} else if (is_src) {
+			block.chunks.hydrate.push(
+				b`if (${element.var}.src !== ${init}) ${method}(${element.var}, "${name}", ${last});`
+			);
+			updater = b`${method}(${element.var}, "${name}", ${should_cache ? last : value});`;
 		} else if (property_name) {
 			block.chunks.hydrate.push(
 				b`${element.var}.${property_name} = ${init};`
@@ -137,7 +143,9 @@ export default class AttributeWrapper {
 			let condition = changed(dependencies);
 
 			if (should_cache) {
-				condition = x`${condition} && (${last} !== (${last} = ${value}))`;
+				condition = is_src
+					? x`${condition} && (${element.var}.src !== (${last} = ${value}))`
+					: x`${condition} && (${last} !== (${last} = ${value}))`;
 			}
 
 			if (block.has_outros) {

test/js/samples/inline-style-unoptimized/expected.js

@@ -15,6 +15,7 @@ function create_fragment(ctx) {
 	let div0;
 	let t;
 	let div1;
+	let div1_style_value;
 
 	return {
 		c() {
@@ -22,7 +23,7 @@ function create_fragment(ctx) {
 			t = space();
 			div1 = element("div");
 			attr(div0, "style", ctx.style);
-			attr(div1, "style", "" + (ctx.key + ": " + ctx.value));
+			attr(div1, "style", div1_style_value = "" + (ctx.key + ": " + ctx.value));
 		},
 		m(target, anchor) {
 			insert(target, div0, anchor);
@@ -34,8 +35,8 @@ function create_fragment(ctx) {
 				attr(div0, "style", ctx.style);
 			}
 
-			if (changed.key || changed.value) {
+			if ((changed.key || changed.value) && div1_style_value !== (div1_style_value = "" + (ctx.key + ": " + ctx.value))) {
-				attr(div1, "style", "" + (ctx.key + ": " + ctx.value));
+				attr(div1, "style", div1_style_value);
 			}
 		},
 		i: noop,

test/js/samples/src-attribute-check/_config.js

@@ -0,0 +1,5 @@
+export default {
+	options: {
+		hydratable: true
+	}
+};

test/js/samples/src-attribute-check/expected.js

@@ -0,0 +1,90 @@
+/* generated by Svelte vX.Y.Z */
+import {
+	SvelteComponent,
+	attr,
+	children,
+	claim_element,
+	claim_space,
+	detach,
+	element,
+	init,
+	insert,
+	noop,
+	safe_not_equal,
+	space
+} from "svelte/internal";
+
+function create_fragment(ctx) {
+	let img0;
+	let img0_src_value;
+	let t;
+	let img1;
+	let img1_src_value;
+
+	return {
+		c() {
+			img0 = element("img");
+			t = space();
+			img1 = element("img");
+			this.h();
+		},
+		l(nodes) {
+			img0 = claim_element(nodes, "IMG", { alt: true, src: true });
+			var img0_nodes = children(img0);
+			img0_nodes.forEach(detach);
+			t = claim_space(nodes);
+			img1 = claim_element(nodes, "IMG", { alt: true, src: true });
+			var img1_nodes = children(img1);
+			img1_nodes.forEach(detach);
+			this.h();
+		},
+		h() {
+			attr(img0, "alt", "potato");
+			if (img0.src !== (img0_src_value = ctx.url)) attr(img0, "src", img0_src_value);
+			attr(img1, "alt", "potato");
+			if (img1.src !== (img1_src_value = "" + (ctx.slug + ".jpg"))) attr(img1, "src", img1_src_value);
+		},
+		m(target, anchor) {
+			insert(target, img0, anchor);
+			insert(target, t, anchor);
+			insert(target, img1, anchor);
+		},
+		p(changed, ctx) {
+			if (changed.url && img0.src !== (img0_src_value = ctx.url)) {
+				attr(img0, "src", img0_src_value);
+			}
+
+			if (changed.slug && img1.src !== (img1_src_value = "" + (ctx.slug + ".jpg"))) {
+				attr(img1, "src", img1_src_value);
+			}
+		},
+		i: noop,
+		o: noop,
+		d(detaching) {
+			if (detaching) detach(img0);
+			if (detaching) detach(t);
+			if (detaching) detach(img1);
+		}
+	};
+}
+
+function instance($$self, $$props, $$invalidate) {
+	let { url } = $$props;
+	let { slug } = $$props;
+
+	$$self.$set = $$props => {
+		if ("url" in $$props) $$invalidate("url", url = $$props.url);
+		if ("slug" in $$props) $$invalidate("slug", slug = $$props.slug);
+	};
+
+	return { url, slug };
+}
+
+class Component extends SvelteComponent {
+	constructor(options) {
+		super();
+		init(this, options, instance, create_fragment, safe_not_equal, { url: 0, slug: 0 });
+	}
+}
+
+export default Component;

test/js/samples/src-attribute-check/input.svelte

@@ -0,0 +1,7 @@
+<script>
+	export let url;
+	export let slug;
+</script>
+
+<img alt="potato" src={url}>
+<img alt="potato" src="{slug}.jpg">